Radiation detector arrays are used to measure the radiation intensity distribution exiting the beam port of a linear accelerator. Such beams generally consist of x-rays or electrons and are used, among other applications, to treat cancer by delivering a lethal dose of ionizing radiation to the tumor. During treatment, the beam port is adjusted to shape the beam to the tumor size. Therefore, precise data on the beam profile is necessary for proper dose control to the tumor. For example, the Profiler, Model 1170 by Sun Nuclear Corp. of Melbourne, Fla., assignee of the subject invention, uses an array of diodes to accomplish a real-time graphic image of the radiation output of a medical accelerator device. The Profiler evaluates the beam flatness, symmetry, field size, and shape.
The linear accelerator is basically an x-ray machine which can produce very high-energy radiation beams and rotates on a gantry for precise delivery to the patient lying on a table. At the heart of the accelerator is an electron gun, which injects a pulse of electrons into an evacuated accelerating tube. The tube is divided into stages through which the electrons are attracted because of a potential difference across each stage. At the end of the tube, the electrons may have been accelerated up to 18 million volts or more. The beam electrons are steered to exit through a thin port as an electron beamor strike a target to produce an x-rays beam. Many factors affect the profile of radiation, including the steering mechanisms, the accelerating voltage, the exit targets, the collimators which shape the beam, and possibly tube sag caused by a change in the gantry angle.
Manufacturers of such accelerators include Varian Corporation in Palo Alto, Calif., who make the Clinac 2100C and Clinac 2500 which produce 2 x-ray beams and several electron beams; and Philips in England, who make the SL25, also a multi energy machine.
When the response of a detector array is tabulated or plotted in a profile of intensity vs detector location, there will be variations in such a profile due to differences in radiation intensity at each location and differences in detector sensitivity at each location. Array calibration is required in order to plot only radiation intensity changes. Calibration of the detector array involves the determination of the relative sensitivity differences between individual detectors in the array. Application of the calibration factors to the measured array response to radiation will result in a profile of the relative radiation intensity at the individual detector locations.
The need for re-calibration will also be determined by the detector type, the beams measured, the frequency of use, and detector service. For example, the sensitivity of diode detectors decreases with large radiation doses. Both ion chamber and diode detectors exhibit an energy response in their sensitivity, therefore calibration at each beam energy will result in improved measurements. The components used to measure any detector output also may change with radiation doses. This will, in effect, change the apparent detector sensitivity which can be corrected with re-calibration. Any time a detector or its measurement electronics is serviced or replaced, the array should be re-calibrated.
Various techniques known to the subject inventors calibrate multiple sensor arrays used on linear accelerators by positioning each detector on the central axis or by using a wide field to cover the array and assumptions are made about exposure reproducibility and or flatness and symmetry.
Wellhofer of Schwarzenbruck, Germany manufactures dosimetry equipment including water tanks which use a single detector that moves about the tank recording the dose in three dimensions, as well as a multi-detector ion chamber array(Model CA24) that simultaneously records the dose at each detector location. The multi-detector array can be calibrated by positioning the array sensors in positions previously measured by the moving single detector. Positioning is accomplished with stepper motors in the tank.
Scanditronix of Uppsala, Sweden manufactures dosimetry equipment including water tanks and multi-detector arrays. Two methods exist for calibrating the array. First, each detector is positioned on the central axis and given the same radiation dose. The second method is to perform the calibration in a wide field, making three measurements, with the center of the array positioned at -25, 0, +25 mm positions and the resulting measurements occur at overlapping positions. Positioning is accomplished with the stepper motors included with the water tank.
Schuster of Forchheim, Germany manufactures a multi-detector array with a central axis calibration procedure using a stepper motor table. Victoreen of Cleveland, Ohio, manufactures a multi-detector array that uses a calibration method which positions each detector at the central axis of the beam, where the calibration is made using a stepper table at the factory.
Various types of patents have also been granted for calibration of radiation type sensors. See U.S. Pat. No. 4,228,515 to Genna et al.; U.S. Pat. No. 4,654,796 to Takagi et al.; U.S. Pat. No. 4,872,188 to Lauro et al.; and U.S. Pat. No. 5,221,842 to Shepherd. These patents will be described below.
Genna ('515) describes the calibration of a detector array used for event position analysis in order to determine radiant emission trajectories emanating from a patient in nuclear medicine imaging. All detectors sample each origin position in order to determine their relative response to that origin, without moving the array. Takagi ('188) describes the calibration of a detector array used in x-ray tomography for the measurement of radiation transmission differences from one radiation exposure to the next, where a change occurred in the absorber. Lauro ('842) describes the calibration of detector arrays to compensate for spatial misalignment of corresponding detectors. The subject is imaging from radiation attenuation signals in the same plane. Shepard ('842) describes the calibration of a detector array which represents quality assurance testing of individual dosimeters. The method of Shepard ('842) assumes the radiation field is uniform at all detector locations which is an invalid assumption in the application of linear accelerator beams. The other prior art patents do not result in a calibration of the detector array for accelerator beams.